1. Field of the Invention
The invention relates to a method of dynamic range compression in an X-ray image, to whose picture elements digital input-picture values are assigned, as well as to apparatus for putting the method into effect.
2. Description of the Related Art
The object areas shown in an X-ray image often have large density differences. In, for example, pictures of a lung, the vertebral column and the heart have significantly higher densities for the X-rays than the lung tissue, i.e. they absorb the X-rays to a significantly greater extent. These density differences in the object result in the X-ray three-dimensional image to be converted by the X-ray image detector into intensity differences up to a factor of 10,000. Conventional X-ray films cannot cope with such a dynamic range without loss in local contrast. However, since one is only interested in a given limited dynamic range--for lung pictures, for example, in the relatively weakly absorbing lung tissue, the films are usually exposed and developed such, that this dynamic sub-range fully utilizes the dynamic range of the film. The other dynamic ranges are shown on the film with overexposure or underexposure, so that the fine contrasts present there are lost.
Contempory image detectors have a much larger dynamic range, so that they are capable of supplying data which can still be utilized as images also in dynamic range boundary regions that cannot be recorded by a conventional film. These detectors include, for example, photo-conductor layers, preferably comprising selenium, which convert the X-ray three-dimensional image into an electric charge pattern, which can be scanned by means of a charge sensor and converted picture element--sequentially into digital input-picture values. Plates coated with storage phosphors, which store the X-ray three-dimensional image as a latent image and, on being irradiated by a laser beam produce fluorescent light proportional to the intensity of the X-ray radiation also have a comparable dynamic range. Using an optical sensor the fluorescent light can be converted into an electric signal, which is subsequently digitized.
If meanwhile the input-picture values thus obtained and containing the overall dynamic range of the object are applied without further processing to a display unit producing an image thereof, then a larger portion of the information still contained in the input-picture values will be lost, when an image character is to be used which corresponds to the character of a "conventional" X-ray recording. It is indeed possible to avoid this loss of information in the areas which are not so relevant for the diagnosis by reducing the contrasts in all the image areas, but this contrast reduction which is also active in the density range which is important for the diagnosis causes a picture character deviating from a conventional recording; the majority of doctors reject such low-contrast ("faint") images.
The EP-A 158 382 which corresponds to U.S. Pat. No. 4,675,893 discloses a method in which the X-ray radiation is varied such that before passing through the area of examination by controllable attenuation elements, the intensity differences behind the examination area and before the image detector are smoothed out to a very large extent. In that case it is also possible to obtain, using a conventional X-ray film, an X-ray image which on the one hand preserves the character (in the average density range) of a conventional X-ray image, but on the other hand enables the display of detail contrasts also in the darker image portions.